Sharper than a saber, lighter than 300/f2.8, smaller! No indication of spherical aberration. No noticeable color fringing at the edge of FOW at 200% magnification

Cons:

Would I dare to say price? Hardly! $4600 is a pretty penny but such a lens commands it!

This feather –light miracle of Canon’s engineering is a straight razor of photography.
March 2010 I had an opportunity to attend a shoot in a remote location. Holy Grail of yesteryears 400/f2.8 (non IS, but who needs IS for a lens that is permanently attached to a tripod) was not in a competition. My other long lens: 300/f2.8, until recently outdoor sport and walk around the park lens just don’t have the reach. Only choices I had was 500/f4 or 400/f4 DO. Day with a borrowed 500 convinced me that boys day over 60 have no business using anything heavier than 300/f2.8. I have not regretted day I purchased EF 400mm /f4 DO. It is SHARP, of great contrast, pleasing color rendering, no aberrations and above all LIGHT. Best wild life lens I have HAND HELD in over 40 years enjoying photography. Final proof that some ill informed individual bumbling about “artifacts” softness and other mirages need to place lens on a tripod and point it in the night sky. Take an image of stars and will notice no comas at the FOW edges. This test separates man from boys and this lens is no boy. This is a very mature lens! Other day I had setup a test image and compared all three great long lenses I am fortunate to own: at 200% pixel peeping 400 DO held its ground (and in the very image corners bettered old 400/f2.8). With 1.4 converter did not give as many more dateless as 300/ f2.8.